This invention relates to a double reverse propeller apparatus used as a propulsion device for a ship.
Conventionally, there is a double reverse revolution propeller apparatus for a ship, as shown in FIG. 3, in which an inner shaft 8 is provided with a rear propeller 7 at its rear end and is connected at its front end directly to the output shaft 1a of a main diesel engine 1 via an intermediate inner shaft 9. An outer shaft 5 is coaxially disposed around the inner shaft 8 and provided with a front propeller 6 at its rear end. The outer shaft is connected at its front end to a reversing device 3' via a hollow shaft 4.
Reversing device 3' is coupled via an elastic coupling 2 to the output shaft 1a of engine 1 and converts torque applied thereto via elastic coupling 2 from output shaft 1a to a rotation in a direction opposite to the direction in which the output shaft 1a rotates and at the same rotational speed as the speed of the output shaft, and transmits such converted rotation to the shaft 4, outer shaft 5 and front propeller 6.
Outer shaft 5 is supported by an outer shaft bearing 15 provided on the side of the hull of the ship while the inner shaft 8 is supported by an inner shaft bearing 16 inserted between the inner and outer shafts 8 and 5.
In FIG. 3, reference numeral 1b denotes a torque branching point, reference numeral 13 an inner shaft thrust bearing, and 14 an outer shaft thrust bearing.
In such dual reverse revolution propeller apparatus, rear propeller 7 receives torque from the output shaft 1a of main diesel engine 1 via intermediate inner shaft 9 and inner shaft 8 and is rotated in the same direction as output shaft 1a. The front propeller 6 receives torque branched from the output shaft 1a via elastic coupling 2, reversing device 3', hollow shaft 4 and outer shaft 5 and is rotated in a direction opposite to that in which rear propeller 7 rotates.
At this time, there are many combinations of the number of blades of propellers 6, 7, engine speeds, and torque distributions. It is said conventionally to be optimal to design front and rear propellers 6 and 7 so that they are rotated in opposite directions at substantially the same rotational speed to produce substantially the same thrust. This is because, when the propellers 6 and 7 are rotated in opposite directions at substantially the same rotational speed, the rotational energy in the flow of the fluid after the front propeller 6 is recovered most efficiently by the rear propeller 7, to thereby improve the propulsion efficiency.
It is to be noted that the thrust generated by the front and rear propellers 6 and 7 are transmitted via outer shaft 5 and inner shaft 8 from outer and inner shaft thrust bearings 14 and 13 to the hull.
That portion of a double reverse revolution propeller apparatus for a ship such as that mentioned above which is most difficult technically to put to practical use is inner bearing 16 which supports inner shaft 8 within outer shaft 5. Inner shaft bearing 16 may be one of various types which include a floating bush type, a hydrostatic bearing type, a roller bearing type, etc. However, it is very difficult to provide inner shaft 16 having a sufficient load capacity between inner and outer shafts 8 and 5 which rotate at equal speeds in opposite directions, even if one of these types of bearings is used. Seizure may occur with high probability.
Consider the load capacity of a bearing in which the outer and inner shafts 5 and 8 are respectively rotating at speeds U2, U1 in opposite directions, as shown in FIG. 5.
(1) Consider the cross section taken along the line 1- 2. By a relative shaft rotation U1+U2, speeds of running fluids (i.e. lubricant) U.sub.Qi and U.sub.Qo are produced on the surface of both the shafts. When outer and inner shafts 5 and 8 rotate at equal speeds in opposite directions (U1 +U2 =0), both the net quantities of forced bearing lubricant oil Qi and Qo obtained by integrating UQi and UQo, respectively, in the radial direction of the shafts become zero.
(2) Since there are zero net quantities of forced oil in the case of equal reverse revolutions, as mentioned above, neither wedge action nor oil film pressure will be produced.
(3) Therefore, since there is no oil film pressure opposing the load of inner shaft 8, metal contact will occur between the inner and outer shafts or between the inner shaft and the bearing therefor and hence seizure may occur.
Pressure distribution in the oil film, as shown in FIG. 5, is theoretically shown by the following Reynolds Equation ##EQU1## where P is pressure, h is spacing distribution, .mu. is oil viscosity, U1 is the peripheral speed of the inner shaft, U2 is the peripheral speed of the outer shaft, x is a circumferential coordinate whose center is the center of the outer shaft, and y is an axial coordinate.
The right side of Equation (1) is called "wedge action". In the case of equal reverse rotations, U1+U2=0. Therefore, there is no wedge action. Therefore, no oil film pressure P obtained by solving the left side of Equation (1) will be produced.
As described above, if a plain bearing is used between outer and inner shafts 5 and 8 which rotate at the same rotational speed in opposite directions, there is no net quantity of oil forced into the spacing between the inner and outer shafts because there is no difference in rotational speed between the outer and inner shafts (there is no difference in peripheral speed between the rotating surfaces of the inner and outer shafts). Thus no "wedge action" of the lubricant will occur, the inner shaft 8 will not float by oil pressure, thereby causing metal contact and hence seizure.
If the inner shaft bearing 16 is seized in such dual reverse revolution propeller shaft system, the shaft system driving of the ship will seriously be influenced, for example, the ship will not be able to navigate.
It could be conceived that if inner shaft bearing 16 is seized, the torque transmitted to reversing device 3' is interrupted, the inner and outer shafts 8 and 5 are tightly fastened so that they rotate in the same direction to thereby prevent an increase in damage due to seizure of the inner shaft 16.
Since the thrusts produced by the front and rear propellers 6 and 7 rotating at the same rotational speeds in opposite directions are equal in the conventional double reverse revolution propeller apparatus, however, they would cancel each other although the front and rear propellers 6 and 7 may be driven in the same direction by fastening the inner and outer shafts tightly. As a result the ship will not be able to navigate.
In the case of an equal-speed reverse rotation system in which the front and rear propellers rotate at substantially the same rotational speed in opposite directions, it is necessary to use parallel shaft gears or a two-stage planetary gear, etc. However, these devices are large-size, complex and expensive.
It is therefore an object of this invention to drive front and rear propellers using a small simple apparatus and to prevent seizure of the inner shaft bearing.
It is another object of this invention to provide a highly practical double reverse revolution propeller apparatus which is cable of producing a thrust and capable of emergency self-navigation even if the inner and outer shafts are tightly fastened and rotated in the same direction even when seizure may occur at the inner shaft bearing.
Further, in the conventional double reverse revolution propeller apparatus, the arrangement is such that the absorption horsepowers of front and rear propellers 41 and 42, as shown in FIG. 4, are usually equal.
Since the propeller torque is proportional to the absorption horsepower/engine speed, and if the front and rear porpellers 41 and 42 are set to be equal in absorption horsepower when they are different in rotational speed, a swirling flow 45 downstream of the propeller as a reaction of the propeller torque would not be cancelled completely, as shown by 45a in FIG. 4 and the energy of such swirling flow accordingly would be lost. It is to be noted that in FIG. 4, reference numeral 43 denotes a flow along the outer end of the propeller, 44 a swirling flow downstream of the front propeller, 46 the direction of rotation of the front propeller, and 47 the direction of rotation of the rear propeller.
This invention is intended to solve the above problems. It is an object of this invention to provide a double reverse revolution propeller apparatus in which the ratio of absorption horsepower of the front propeller to that of the rear propeller is equal to the ratio of rotational speed of the front propeller to that of the rear propeller to cancel the swirling flows of the front and rear propellers, to decrease loss of the rotating energy by the propellers and to thereby improve the propulsion efficiency of the ship.
In the conventional propeller apparatus shown in FIG. 4, the diameter of rear propeller 42 is designed so as to contact flow 43 along the outer edge of front propeller 41, while in the double reverse revolution propeller apparatus in which the front and rear propellers 41, 42 are equal in rotational speed, the front propeller 41 has a smaller number of blades than the rear propeller 42. In this case, if the number of blades of the respective propellers is selected incorrectly, the swirling flows downstream of the propeller will remain not completely cancelled.
This invention is intended to solve this problem. It is an object of this invention to provide a dual reverse revolution propeller apparatus which has a simplified reverse revolution mechanism for the propeller shaft while cancelling the swirling flows downstream of the front and rear propellers sufficiently to reduce loss of the rotating energy of the propellers, thereby improving the propulsion efficiency of the ship.